We asked Shaykh ‘Abd-Allaah ibn ‘Abd al-Rahmaan al-Jibreen (may Allaah preserve him) and he replied as follows:
The pilgrims who are camped in Muzdalifah in the camp that contains the overflow from Mina come under the same ruling as those who are in Mina. If they want to hasten on, they should leave their places before Maghrib. It is permissible for them to leave if they have prepared to leave before Maghrib and are delayed by the crowds, as is the case with the people of Mina if they prepare to leave and resolve to do so but are delayed by the crowds.
As for those who stayed in various parts of Mina, such as some areas of Makkah, because they cannot find any room in Mina, it is sufficient for them to have the intention of hastening on, but they should have stoned the Jamaraat and left the boundaries of Mina before Maghrib. If they are delayed by the crowds, they may still hasten on as stated above.
And Allaah knows best.